Keywords
clinopyroxene
EPMA
How to Cite
Abstract
High-precision estimates of Fe valence (Fe3+/ΣFe, where ΣFe = Fe2++Fe3+) in glasses and isotropic minerals from Fe K-edge X-ray absorption near-edge structure spectroscopy (Fe-XANES) have greatly improved our understanding of magmatic fO2 in recent years. However, isotropic phases are not always present in the rock record, and our poor understanding of Fe3+/ΣFe in anisotropic minerals, including near-ubiquitous clinopyroxene, hampers our ability to use them to investigate magmatic fO2. Here we evaluate strategies for using pre-edge centroid positions obtained from Fe-XANES to determine Fe3+/ΣFe in clinopyroxene powders and oriented single crystals. First, we show that clinopyroxene Fe3+/ΣFe can be calibrated against pre-edge centroid positions collected from powdered reference materials characterised by Mössbauer spectroscopy, albeit with a precision of 11% (1σ absolute). Second, spectra collected from oriented crystals reveal that centroid positions depend not only on crystal orientation but also that the nature of this dependence varies with Fe3+/ΣFe. Nevertheless, we are able to determine Fe3+/ΣFe in unknown, but oriented, clinopyroxene crystals with precisions of 12–19% (1σ absolute). Applying clinopyroxene Fe-XANES to samples from Iceland and the Azores validates previously reported estimates of Fe3+/ΣFe from stoichiometry. However, our findings confirm that determining clinopyroxene Fe3+/ΣFe by Fe-XANES requires reference materials and unknowns to be reproducibly oriented to within a few degrees, a necessity that makes Fe-XANES ill-suited for routine analyses of clinopyroxene. We find that electron microprobe-based approaches readily and rapidly return more precise clinopyroxene Fe3+/ΣFe determinations than the Fe-XANES approaches we describe here, and are hence more appropriate for measuring the large numbers of samples required to investigate the nature and causes of fO2 variability in magmatic systems.
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Copyright (c) 2026 David A. Neave, Elisabetta Mariani, Alexander G. Stewart, Margaret E. Hartley, Oliver Shorttle, Madeleine C. S. Humphreys